1. Field of the Invention
The present invention relates to apparatus for and methods of remotely measuring the temperature of a specular surface, and has particular utility in the field of laser thermal processing (LTP) where a beam of radiation is scanned across a substrate surface to heat the surface, and has general utility to thermal processing where the temperature of a specular surface needs to be known to a relatively high degree of accuracy and precision.
2. Description of the Prior Art
LTP is a technique for manufacturing semiconductor devices such as integrated circuits or “ICs”. LTP involves irradiating a substrate, such as a doped semiconductor wafer, to rapidly bring the substrate surface from a relatively low temperature (e.g., 400° C.) to a relatively high temperature (e.g., 1,300° C.) quickly so that the substrate bulk can pull the temperature down quickly. Such a rapid thermal cycle might be used, for example, to efficiently activate dopants in the substrate because only the material very close to the top surface of the substrate is heated to the relatively high temperature during irradiation.
As described in U.S. Pat. No. 6,747,245, one approach to LTP involves scanning a long, narrow laser beam back and forth across the wafer surface in a raster pattern. The amount of time the laser beam resides over a given point on the wafer surface is called the “dwell time”. Using this scanning approach, it is possible to achieve peak surface temperatures near 1350° C. with dwell times in the millisecond range. The result is a rapid thermal annealing of doped wafers that yields high activation levels with very little dopant diffusion. When used to fabricate transistor-based circuits, transistors with a sharply defined dopant profile and with a small sheet resistance are formed. LTP is capable of providing significantly lower sheet resistance values than possible using so-called Rapid Thermal Processing (RTP), which has dwell times equivalent to several seconds.
While the LTP process is quick and effective, the results are sensitive to the peak temperature produced on the substrate surface. A difference in the peak annealing temperature of as little as a 5° C. can result in a measurable and undesirable difference in sheet resistance.
Accordingly, it would be advantageous to be able to measure the substrate surface temperature during LTP. Because of the rapid heating and cooling cycle associated with LTP, and because of the scanning geometry, in practice the peak surface temperature is best measured remotely. The most robust prior-art remote surface-temperature measurement techniques require making measurements of emitted radiation made at a number of different wavelengths. The ratios of these measurements are used to make a number of different temperature estimates. These estimates are then weighted and combined to reach a final estimated temperature. Unfortunately, this approach is relatively complex and not sufficiently accurate for the range of temperatures and the variations in surface emissivity associated with LTP.